As the overall dimensions of semiconductor devices continue to shrink, the demand is ever increasing for forming more metal interconnection layers on a semiconductor substrate. The metal interconnection layers or inter-layers for insulating the metal interconnection layers should be planarized to secure a focus margin for a following photolithography process. In most conventional fabrication processes the insulating inter-layers have been planarized using a BPSG (borophsophosilicate glass) reflow technique. In the BPSG reflow technique, a deposited BPSG layer is planarized by flowing the BPSG layer while heating the substrate to 800.degree. C. or more. However, the temperature of the reflow process is too high to be applicable to aluminum interconnection layers. In addition, the reflow technique is inadequate for the planarization of sub-micron fine patterns or the global planarization of a substrate.
Instead of the reflow technique, a resist etchback technique is known in the art of surface planarization. The resist etchback technique however has several drawbacks such as the increase in the thickness of insulating inter-layers, the necessity of additional processes and the difficulty of controlling etch ratio of insulating inter-layers and resist layers.
Therefore, recently chemical mechanical polishing (CMP) has been developed for providing planarized topographies. CMP is a process for improving the surface planarity of a semiconductor substrate and involves the use of a polishing pad with a slurry. Generally, the slurry contains a mechanical polishing agent, such as alumina or silica. Additionally, the slurry contains de-ionized water and selected chemicals which etch various surfaces of the substrate during processing. The chemicals include pH controlling solution, such as KOH or NaOH. The mechanical polishing and chemical polishing are simultaneously performed during overall polishing process.
in the CMP process the polishing rate is proportional to the parameters such as the pressure of the polishing pad on the substrate, and the relative velocity between the substrate and the polishing pad. In case of rotational motion, the perimeter of the substrate tends to rotate at higher velocity than the center of the substrate. Therefore in the CMP process used for semiconductor device fabrication, the relative velocity between the substrate and the polishing pad is maintained to be equal by combining two rotational movements with same rotational velocities.
FIG. 1 is a partial sectional view of a CMP apparatus in which the conventional CMP process is employed. Referring to FIG. 1, a platen drive motor 130 rotates a polishing pad 110 attached to a platen 120. A substrate 140 retained in a substrate carrier 150 is rotated against the pad 110 by a carrier drive motor 160. When the substrate 140 and the pad 110 rotate with a slurry supplied to the interface of the substrate 140 and the pad 110, the substrate 110 is chemically and mechanically polished.
The relative motion between a substrate and a polishing pad is shown in FIG. 2 when they respectively rotate. At an arbitrary point P on the substrate, the moving velocity V.sub.p1 pad of the substrate relative to the polishing pad can be represented by equation 1, that is, a difference between substrate moving velocity V.sub.PH and pad moving velocity V.sub.PP. EQU V.sub.p1 pad =V.sub.PH -V.sub.PP (equation 1)
Since the velocities V.sub.PH and V.sub.PP are equal to the vector products of the respective angular velocities and radius vectors, equation 1 can be expressed as following equation 2. EQU V.sub.p1 pad =V.sub.PH -V.sub.PP =.omega..sub.H.times.r.sub.H -.omega..sub.P.times.r.sub.P =.omega..sub.H.times.r.sub.H -.omega..sub.P.times. EQU (r.sub.cc +r.sub.H)=(.omega..sub.H -.omega..sub.P).times.r.sub.H -.omega..sub.P.times.r.sub.cc (equation 2)
where .omega..sub.H is the rotational angular velocity of the substrate, .omega..sub.P is the rotational angular velocity of the polishing pad, r.sub.H is the position vector from the substrate rotation center to the point P, r.sub.P is the position vector from the polishing pad rotation center to the point P, and r.sub.cc is the position vector from the polishing pad rotation center to the substrate rotation center.
If the rotation angular velocities of the substrate and the polishing pad are equal (.omega..sub.H -.omega..sub.P), the equation 2 reduces to equation 3. EQU V.sub.p1 pad =-.omega..sub.P.times.r.sub.cc (equation 3)
From the equation 3, it will be appreciated that the relative moving speed between the substrate and the polishing pad depends upon the rotational angular velocity and the distance between two rotation centers, but not the position or direction on the substrate.
Accordingly, if the respective drive motors of the platen and the carrier are controlled such that the rotational angular velocities of the substrate and the polishing pad may be equal, the substrate can be uniformly polished since the relative moving speed between the substrate and the polishing pad is equal at all points on the substrate.
The movement of the equation 3 is equivalent to that a substrate orbits with a radius of r.sub.cc against a stationary polishing pad without rotation, as shown in FIG. 3A to FIG. 3C. From the viewpoint of relative motion, it is also equivalent to that a polishing pad orbits with a radius of r.sub.cc against a stationary substrate without rotation. FIG. 3A shows sequentially the states that the substrate and the polishing pad are rotated by 0, 45, 90, 180 and 270 degrees, respectively. Black dots are inserted in the figure to indicate the absolute positions of the substrates and the polishing pad. In case of fixing the position of the polishing pad, FIG. 3A can be expressed by FIG. 3B. As shown in FIG. 3C, all points of the substrate trace a circular path with a radius of r.sub.cc on the polishing pad.
If a point of a substrate traces a line path on a polishing pad, any trivial non-uniformity on the polishing pad can exert a harmful effect upon the substrate. Therefore, the area which a point of a substrate traces should be widened. For this reason, it is desirable that a substrate carrier or a platen is reciprocated within a predetermined range with a relatively low speed, as a sub motion. Hereinafter, the movement which induces chemical-mechanical polishing effect will be referred to as "main motioned", whereas the movement, making little contribution to the polishing, for obtaining other effects will be referred as "sub motion". In prior art CMP method, the rotational motions of the substrate and the polishing pad are examples of the main motion.
FIG. 4A shows sequentially the states that the substrate and the polishing pad are rotated by 0, 45, 90, 180 and 270 degrees, respectively, in case of combining uniform-velocity rotational motions of the substrate and the polishing pad with their low speed reciprocating motions, as sub motions.
Short lines are inserted in the figure instead of black dots of FIG. 3A to indicate the absolute positions of the substrates and the polishing pad, which represents that the tracing area is widened in the direction of reciprocating motion. In case of fixing the position of the polishing pad, FIG. 4A can be expressed by FIG. 4B. The area which an arbitrary point of a substrate traces is widened as shown in FIG. 4C. In addition, all points of the substrate trace the same area and the tracing paths are uniformly spread in all directions of the polishing pad. In this case, it is desirable that the ratio of the period of rotation movement to that of reciprocating movement is not a simple integer. If the ratio is a simple integer, the points of the substrate trace a closed curve path within the donut shape of FIG. 4C. Most ideally, if the ratio is not an integer but an irrational number, their tracing paths will completely cover the inside of the donut shape.
However, in a prior art CMP method, which respectively employs uniform-velocity rotational motions of the substrate and the polishing pad as main motion and their low speed reciprocating motions as sub motion, additional drive means for the sub motion is required instead of two drive motors for the rotations of the substrate and the polishing pad, resulting in complex mechanical configuration. Moreover, the rotation speeds of the two drive motors should be controlled to be within extremely small error range in order to maintain polishing uniformity at all points of the substrate.
U.S. Pat. No. 5,554.064 discloses a CMP method that combines a non-rotational orbiting of a polishing pad (,main motion) with a low speed rotation of a substrate (sub motion). In addition. U.S. Pat. No. 5,582,534 discloses methods and apparatus for CMP that combines a rotation of a polishing pad (main motion) with an orbiting of a substrate (sub motion). However, in the above methods, those areas of the substrate which are located further from the rotation center experience greater cumulative movement, and therefore greater material removal, than areas of the substrate maintained closer to the rotation center.